The present invention relates to a pickup unit for use in an optical record/reproduce system.
Video disks, digital audio disks and other information recording mediums in the form of disk store information signals in tiny pits (or dips) that are etched onto the disk surface in the form of a spiral track in correspondence with the information signals to be recorded. In order to read the recorded information signal, a small beam spot is emitted on the surface of the disk and the change in the reflected light which is determined by the presence of or absence of a pit is converted to a corresponding electrical signal for reproduction of the original information signal. Faithful signal reproduction requires the emitted light to be correctly focused on the recording surface of the disk and, to this end, the position of the objective lens for focusing the emitted light on the recording surface must be controlled in the direction of its optical axis (focusing direction) by using a focusing servo drive. In addition to this need for performing focus servo, the emitted light must follow the recording track accurately by virtue of its position being controlled in the tracking direction normal to the optical axial direction. This tracking servo and the focusing servo drive must be supplemented by the "tilt servo" which is necessary to maintain the recording surface of the disk in an orthogonal relationship with the optical axis of the emitted light. If the orthogonal relationship between the recording surface of the disk and the optical axis of the emitted light is not established, the beam spot focused on the recording track being read becomes distorted and crosstalk (i.e., interference from information recorded on an adjacent recording track can cover). The tilt servo is necessary in order to avoid this problem.
While various mechanisms for performing the tilt servo have so far been developed, they are essentially the same in that they incorporate a sensor for detecting the angle of relative inclination between the recording surface of the disk and the optical axis of the emitted light, with the sensor being in most cases composed of a single light-emitting device and two light-receiving devices. This sensor is attached to a sensor board mounted on a holder mechanism which holds a plurality of optical elements including the objective lens mentioned above and which is movable on the recording surface of the disk.
It is not however necessarily easy to ensure precise detection of the angle of relative inclination between the recording surface of the disk and the optical axis of the emitted light because of such factors as the errors introduced into the radiation mode and the amount of light issued from the light-emitting device, and variations in the output characteristics of the light-receiving device.
The pickup unit is typically equipped with a control circuit board which ensures proper performance of the aforementioned three servo drives, i.e., focus servo, tracking servo and tilt servo. This control circuit board is fixed to the main frame of a retainer mechanism by means of screws, washers and associated devices, with an intervening element such as a positioning member being disposed between the circuit board and main frame. This is also true with the sensor board to which the aforementioned sensor is attached. In other words, the conventional pickup unit includes a sensor board, control circuit board and other individual components which are mounted on their retainer mechanism by means of screws and other appropriate fastening means. Not only does this lead to high manufacturing costs because of the need for using a large number of parts but also the overall size of the pickup unit is increased.